A Group III nitride semiconductor has a direct transition-type band gap corresponding to the region from visible light to ultraviolet light and can realize highly efficient light emission. Therefore, it is formed into an LED or LD. Furthermore, for example, a two-dimensional electron layer appears on the hetero junction interface between aluminum gallium nitride (AlGaN) and gallium nitride (GaN) due to the piezoelectric effect characteristic of the Group III nitride semiconductor. Thus, there is a potential of providing an electronic device having properties that are not obtained by conventional Group III-V compound semiconductors.
However, the Group III nitride semiconductor is difficult to grow as a single crystal because the dissociation pressure of nitrogen reaches as high as 2,000 atm at the growth temperature of the single crystal. Therefore, unlike Group III-V compound semiconductors, a single crystal substrate of the Group III nitride semiconductor cannot be used at present as the substrate for use in the epitaxial growth. As the substrate for use in the epitaxial growth, a substrate composed of a heterogeneous material, such as sapphire (Al2O3) single crystal or silicon carbide (SiC) single crystal, is used.
Between such a heterogeneous substrate and the Group III nitride semiconductor crystal epitaxially grown on the substrate, a large lattice mismatch is present. For example, there is present a lattice mismatch of 16% between sapphire (Al2O3) and gallium nitride (GaN), and that of 6% between SiC and gallium nitride. When such a large lattice mismatch is present, it is generally difficult to epitaxially grow a crystal on the substrate. Even if grown, a crystal having good crystallinity cannot be obtained. Therefore, in the case of epitaxially growing a Group III nitride semiconductor on a substrate of sapphire single crystal or SiC single crystal by the metal-organic chemical vapor deposition (MOCVD) method, a method of first depositing a low-temperature buffer layer composed of aluminum nitride (AlN) or AlGaN on the substrate and then epitaxially growing a Group III nitride semiconductor crystal thereon at a high temperature is generally employed, as disclosed in Japanese Patent 3,026,087 and JP-A HEI 4-297023 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
Other than the growth method using a low-temperature buffer layer, a method of growing and thereby forming an AlN layer on a substrate at a high growth temperature on the order of 900 to 1,200° C. and then growing gallium nitride thereon is disclosed, for example, in Applied Physics Letters, P. Kung, et al., 66, 2958 (1995) and JP-A HEI 9-64477.
In the case of using sapphire as the substrate, the above-described low-temperature buffer layer is generally formed as follows.
First, the sapphire substrate is heated to a high temperature of 1,000 to 1,200° C. in a growing apparatus for the MOCVD method to remove the oxide film or the like from the surface. Then, the temperature of the growing apparatus is lowered and at a temperature on the order of 400 to 600° C., a low-temperature buffer layer is deposited on the substrate by simultaneously supplying a raw material organic metal and a nitrogen source to give a V/III ratio of 3,000 to 10,000. Here, the V/III ratio is a ratio of the molar number of molecules containing the Group III element and that of molecules containing the Group V element, which are passing in the reaction furnace at the time of growing a Group III-V compound semiconductor crystal by the MOCVD method. For example, in the case of growing gallium nitride using TMGa and ammonia, this ratio is a ratio of the molar number of TMGa and that of ammonia, which are passing in the reaction furnace. Thereafter, the supply of the raw material organic metal is stopped, the temperature of the growing apparatus is again elevated to perform a heat treatment called crystallization of a low-temperature buffer layer, and then the objective Group III nitride semiconductor crystal is epitaxially grown.
At a temperature of 400 to 600° C. that is a temperature for depositing the low-temperature buffer layer, thermal decomposition of the raw material organic metal and nitrogen source used as starting materials, particularly ammonia used as the nitrogen source, proceeds insufficiently. Accordingly, the low-temperature buffer layer deposited at such a low temperature and left intact contains many defects. Furthermore, since the starting materials are reacted at a low temperature, a polymerization reaction occurs between the alkyl group of the raw material organic metal and the undecomposed nitrogen source, and the reactant thereof and other impurities are also contained in a large amount in the crystal of the low-temperature buffer layer.
For eliminating these defects and impurities, a heat-treating step called crystallization of a low-temperature buffer layer is performed. The step of crystallizing the buffer layer includes heat-treating the low-temperature buffer layer containing many impurities and defects at a high temperature dose to the epitaxial growth temperature of the Group III nitride semiconductor crystal to thereby remove the impurities and defects.
As such, in the growth method using a low-temperature buffer layer, the substrate temperature must be lowered from 1,200° C. as a temperature for thermal cleaning to around 500° C. as a temperature for growing a buffer layer and subsequently elevated from around 500° C. to a temperature region close to 1,000° C. as an annealing temperature within a relatively short time. At this time, generally, the change of temperature accompanying the cooling takes a long time and the abrupt increase of the temperature requires a large amount of energy.
Furthermore, these various temperature histories given to the substrate cause warping of the substrate, and the warped substrate may be cracked or crazed. Also, the warping of the substrate affects the crystal layer grown thereon and particularly in the manufacture of an LED structure, the emission wavelength or emission intensity becomes inhomogeneous in the substrate plane.
Other than such a growth method using a low-temperature buffer layer, a method of growing and thereby forming AlN on a substrate at a high growth temperature on the order of 900 to 1,200° C. and then growing gallium nitride thereon is disclosed (see, for example, Applied Physics Letters, P. Kung, et al., 66, 2958 (1995)). In this publication, it is stated that a very good crystal of 30 arc sec as the X-ray rocking curve of the (0002) plane can be produced by this method. However, according to the test performed by the present inventors, the gallium nitride crystal film produced by this technique is found to be a crystal having very high columnarity and contain a large number of grain boundaries within the crystal. Such a crystal is high in the density of threading dislocations generated from the substrate toward the surface. Therefore, even if a device structure, such as light-emitting device and electronic device, is manufactured, good properties cannot be obtained.
The growth method using an AlN layer produced at high temperature is also described in JP-A HEI 9-64477. This patent publication states that the manufactured Group III nitride semiconductor crystal is preferably a single crystal having good crystallinity. Though the present inventors conducted repeated tests, a crystal capable of providing a device structure having good properties could not be grown by the growth method using a good single crystal AlN film described in this patent publication. This is considered to occur because when a single crystal layer having good crystallinity is used as the buffer layer, migration of atoms attached does not proceed successfully at the initial stage of growth on growing a Group III nitride semiconductor thereon and the two-dimensional growth hardly takes place.
As such, a Group III nitride semiconductor crystal having crystallinity high enough to produce a device cannot be obtained and the method of growing a Group III nitride semiconductor crystal using an AlN buffer layer grown at high temperature is not commonly used at present.
One object of the present invention is to provide a method for producing a Group III nitride semiconductor crystal, which can take the place of the method using a low-temperature buffer layer and requiring setting of many temperature regions or the method using a high-temperature AlN layer and having a problem in the crystal quality, and can form a high-quality Group III nitride semiconductor crystal through steps relatively reduced in the change of temperature.
Another object of the present invention is to provide a method for producing a Group III nitride semiconductor crystal, which enables a high-quality Group III nitride semiconductor crystal to be epitaxially grown on a sapphire substrate.
Still another object of the present invention is to provide a Group III nitride semiconductor epitaxial wafer that can advantageously be used for an LED, electronic device or similar device.